BACKGROUND: There has been a debate, whether mild or moderate ischemic mitral regurgitation (IMR) should be repaired at the time of coronary bypass surgery. We assessed the early and midterm outcomes of patients with mild or moderate IMR who underwent isolated off-pump coronary artery bypass grafting (OPCAB). METHODS: We retrospectively reviewed records of 602 patients who underwent OPCAB between April 2001 and July 2004. Of those, 96 patients had mild or moderate MR (IMR group), and 506 had none or trivial MR (no IMR group). Preoperative and postoperative characteristics were compared between the groups. Furthermore, fates of grade of MR and functional status were investigated in patients with IMR. RESULTS: The preoperative mean New York Heart Association (NYHA) classes were 2.7 +/- 1.0 and 2.4 +/- 0.9 in the IMR and no IMR groups, respectively (p < 0.01). Operative mortality was similar between groups (1.0% in the IMR group and 0.4% in no IMR group, p = 0.41). The cumulative survival rate for the IMR group was 93.7% at 1 year and 90.1% at 4 years. The mean NYHA class was 1.1 +/- 0.4 (p < 0.0001) at 28.8 +/- 11.4 months after operation for the IMR group. The mean grade of mitral regurgitation diminished (from 2.2 +/- 0.4 to 1.1 +/- 0.6, p < 0.0001) soon after operation in patients with IMR, and remained so (1.1 +/- 0.8, p < 0.0001) during the follow-up period. CONCLUSIONS: Mild or moderate IMR could be improved with isolated OPCAB. The early and midterm functional outcomes of these patients were favorable.